1. Field of the Invention
This invention relates to the wire bonding technology in general. More specifically, the present invention relates to a method of forming a ball at a lower end of a vertically arranged material solder wire for conveniently bonding to a workpiece such as a semiconductor chip and/or a leadframe.
2. Description of the Related Art
In electronic circuits or components, it is usually necessary to electrically connect between two different portions. For instance, a semiconductor chip need be electrically connected to a relevant lead of a leadframe through a metal wire.
In wire bonding, generally, a gold or silver wire is melted at both ends to form balls which are subsequently utilized for strong attachment to two different bonding portions. Such a wire bonding method, which is called "ball bonding", is possible because either gold or silver has a high melting point and is reluctantly oxidized.
If the melting point of a metal wire is low, it is difficult to control ball formation (with respect to the ball diameter for example) at the time of thermally cutting a material metal wire by a torch flame (formed by burning of hydrogen gas in the presence of oxygen for example). Further, if the metal wire is easily oxidized at the time of ball formation, the metal wire fails to provide a sufficiently strong and reliable connection to the relevant bonding portion.
On the other hand, it has been recently proposed to use a solder wire for wire bonding in place of a gold or silver wire, because solder is known to be less costly than either gold or silver. Further, a solder wire is also advantageous in that it can be made to have an additional function as a temperature fuse (or as a combined temperature/overcurrent fuse) since the melting point of solder is relatively low.
However, the low melting point (e.g. 296.degree. C.) of solder makes it difficult to control ball formation at both ends of a solder wire. Particularly, when the solder wire is heated by a torch flame having a high temperature of about 2,000.degree. C. for example, the lower end of the solder wire is melted too quickly in a large amount, so that the melted solder portion may likely to fall under gravity or otherwise deformed into an oblong form. Further, solder is known to be oxidized very easily at the time of melting. Thus, it has been believed impractical to applying the ball bonding method to the solder wire.
It is conceivable to form a ball at each a solder wire in an oxygen-free atmosphere in an attempt to prevent oxidation of the ball. However, since a torch flame is formed by combustion of a fuel gas (e.g. hydrogen) in the presence of excess oxygen, water vapor as a combustion product and/or excess oxygen inevitably oxidizes the ball end if the flame is made to directly act on the solder wire.
In a wire bonding method using a solder wire, therefore, two different methods are conventionally used for bonding the solder wire. A first method is the soldering method wherein a separate bonding solder layer is formed between each end of the solder wire and a relevant portion of a workpiece. A second method is the so-called "wedge bonding" method wherein each end of the solder wire is compressed by a bonding tool for flattening at the time of bonding to the workpiece.
However, the soldering method requires the use of solder in addition to the solder wire itself to result in a material waste. More importantly, the wire bonding process cannot be performed quickly and efficiently by the soldering method. Further, the soldering method cannot be applied when the interval between the bonding points is narrow.
On the other hand, the wedge bonding method has no problem of material waste and is applicable even when the interval between the bonding points is narrow. However, compared with the ball bonding method, there is a greater limitation in increasing the bonding area by flattening the wire end, so that it is difficult to obtain a sufficient bonding strength. Further, the flattened end portion of the wire is rather thin and easy to break, resulting in a quality deterioration of the products.